The present invention relates in general to a new and useful ceramic fiber composite structure and a method of making same and, in particular, to a ceramic fiber composite filter useful for hot gas cleanup applications which employs a distribution of continuous ceramic fiber and chopped ceramic fibers throughout the filter which results in a unique, lightweight construction having improved strength and toughness.
Furnace exhaust gases resulting from the combustion of fossil fuels typically contain many impurities. Filters have been used to reduce or eliminate the impurities in such furnace exhaust gases. However, there is an increased need for improved filters which are capable of withstanding higher temperatures and pressures for removing particulates from hot combustion gases.
Advanced coal-fired power generation systems such as integrated gasification combined cycle (IGCC) or pressurized fluid bed combustor (PFBC) based systems rely on hot gas filtration equipment to meet turbine inlet gas stream requirements and clean air requirements. In the case of the pressurized fluidized bed combustor (PFBC), the combustion gas stream is provided at a temperature of approximately 1600.degree. F. (871.degree. C.) and it contains both coal ash and fluidized bed material.
In order to maintain system efficiency, it is necessary that the filter system operate at or close to the combustion temperature. Both metal and ceramic tubular or candle (closed-end tube) filters are used to separate the particulates from the gas stream at elevated temperature. Tube filters and candle filters both utilize rigid ceramic filter elements. In tube filter systems, the ceramic filter element is mounted between tube sheets and the gas flows from the inside to the outside of the element. In candle filter systems, the dirty gas is on the outside of the filter element and clean gas flows into the inside of the element. Ash accumulations on the filter surface are removed by back-pulsing with high pressure gas applied in the reverse flow direction at regular intervals ranging from a few minutes to a few hours. Since the back-pulse gas may be at ambient temperature or only slightly pre-heated, the back-pulse process subjects the filter material to a significant thermal transient condition. In addition, unpredictable system upsets, such as combustor or turbine trips, may result in even more severe thermal transients. Filter failures have been attributed to the accumulation of damage caused by these types of thermal transients. Finally, the hot gas filters must also be resistant to the corrosive effects of any alkali, sulfur, and steam components contained within the hot combustion gas stream.
The ceramic filter elements are typically made of a dense, coarse-grained refractory material, such as Cordierite or silicon carbide, and bonded with a second phase. The bond phase may be crystalline or glassy and it is critical to the strength and corrosion resistance of the filter. The open porosity of such monolithic filter materials is approximately 40 percent. The bend strength of monolithic filter materials is in the range of 1 to 4 ksi (ksi=1000 lbs-per square inch). The fracture behavior is brittle which typically results in catastrophic failure of the filter element.
Various ceramic fiber-based filter elements are under development. These include vacuum formed chopped ceramic fiber filters, chemical vapor infiltration (CVI) bonded ceramic fiber filters, and sol-gel bonded continuous ceramic fiber filter elements. One type of filter is produced by vacuum forming chopped or discontinuous ceramic fiber on a mandrel using standard non-woven or felt production methods. The vacuum formed chopped ceramic fiber preforms are impregnated with aluminum oxide and/or silicon dioxide colloidal solutions and heat treated to develop a bond at the fiber contact points. The resulting filter element exhibits bend strengths in the range of 200 to 500 psi (pounds per square inch). Higher strength is required to meet the thermal and mechanical demands of this application. The filters composed of continuous ceramic fibers bonded by chemical vapor infiltration silicon carbide exhibit acceptable strength, but may not be chemically stable in the oxidizing or other corrosive environments of a pressurized fluidized bed combustion system.
A similar type of high temperature ceramic composite filter is disclosed in U.S. Pat. No. 5,196,120 to White. The ceramic fiber filter is useful in filtering gases at elevated temperatures, such as those from a furnace, and the filter is composed of ceramic fibers coated with an intermediate carbonaceous layer and an outer coating of silicon carbide using chemical vapor deposition. In one embodiment, the filter has a rigid preform base of continuous ceramic fiber strands. Discontinuous fibers may be applied as an optional step following production of the preform, by immersing the preform in a slurry tank and creating a vacuum inside the preform to attract the discontinuous fibers (see Col. 3, line 52 to Col. 4, line 5). White thus teaches that a vacuum wound preform coated with a slurry of chopped fiber is known. Instead, the preform and discontinuous fibers are treated with phenolic resin, and then heated in successive steps to cure and bind the fibers together. Further, White also indicates that winding of the preform and the coating with chopped fiber slurry are two distinct steps, rather than one simultaneous, continuous step. The resulting preform is therefore comprised of separate layers of continuous fibers and chopped fibers.
Singh et al. (U.S. Pat. No. 5,407,734) teaches a ceramic fiber composite laminated tape. The tape is composed of a layer of fibrous material, with spaces between the fibers, which is coated with a slurry of ceramic fiber whiskers and organic binding material. Several tapes are then combined and laminated to form the composite tape. The laminated tape is then heated to drive off the organic binding material and hot pressed to form the composite. Singh et al. discloses the use of silicon and aluminum compounds for making the ceramic fiber. Singh et al. also discusses prior slurry coating techniques and bonding techniques. In col. 1, lines 16-35 of Singh et al. states that passing a filament through slurry does not attract enough matrix components, and that chemical vapor deposition methods of binding the materials is too slow.
Farris et al. (U.S. Pat. No. 5,102,601) teaches fabricating a composite by extruding a viscous fiber and passing the fiber through a water bath to cause coagulation of the fiber material before winding it on a take-up roll. This teaching of this patent is distinct from the present invention in that it does not coat a fiber with slurry in the tank, but is instead using the tank to cure the fiber material.
Stinton et al. (U.S. Pat. No. 5,075,160) discloses a filter for removing particulate matter from high temperature flowing fluids, particularly gases, that is reinforced with ceramic fibers and coated with a thin layer of a protective and bonding refractory applied by chemical vapor deposition (CVD) techniques. A thin and extended layer of a ceramic felt, paper, etc. forms the preform which is coated with the ceramic, and is advantageously silicon carbide (SiC).
The use of chemical binders with some ceramic fiber and metal composites is disclosed in an article by Jeng-Maw Chiou and D.D.L. Chung, entitled, "Improvement of the temperature resistance of aluminum-matrix composites using an acid phosphate binder--Part 1--Binders", appearing in the Journal of Materials Science 28, p.1435-1446, .COPYRGT. 1993 Chapman & Hall. The article discusses various binder compositions, such as silica and phosphate binders, and their uses in forming ceramic-metal matrix compositions.
Eggarstedt, of Industrial Filter & Pump Mfg. Co., Inc. discusses work performed under DOE contract DE-FG02-92ER81349 from Jul. 22, 1992 to Feb. 17, 1995 in a paper entitled "IF&P Fibrosic.TM. Filters". The paper discloses formation of ceramic filter elements using vacuum formed chopped ceramic fiber. However Eggarstedt does not disclose any simultaneous application or use of continuous ceramic fibers in addition to the chopped fiber, in contrast to the present invention.
There remains a critical need to develop a more rugged hot gas filter tube material in order to improve the reliability of the aforementioned advanced energy systems.